Electrostatic pigment filter

ABSTRACT

Method and apparatus for removing solid particles from a suspension by moving the suspension through a filter in which an electrostatic charge is generated. In one embodiment the electrically sensitive solid particles are attracted to and held by a dielectric porous filter material while an electrically insulating fluid carrier continues through the filter housing where it exits free from solid particle contamination.

[151 3,655,550 [451 Apr. 11,1972

United States Patent Davies [54] ELECTROSTATIC PIGMENT FILTER 3,182,803 5/1965 Chisho1m................. ....210/266 3,252,885 6/1966 Griswo1d................. ....204/302 ....204/302 [72] Inventor:

Terence Davies Santa Barbara 3,398,082 8/1968 Lochmann et a1. Xerox Corporation, Rochester, NY.

Mar. 12, 1969 [73] Assignee: 3,413,988 12/1968 Butler....................................134/102 [22] Filed:

Primary ExaminerF. C. Edmundson Attorney-James J. Ralabate, Barry Jay Kesselman and David C. Petre [21] Appl. No.: 806,637

it exits free from solid particle contamination.

1,831,075 11/1931 Nee1ey................ ..........204/302X 2,640,026 5/1953 Whittington...........................204/302 l3Claims,4DrawingFigures PATENTEDAPR 1 1 I972 SHEET 2 BF 3 oo0ooooooooooof/40 PATENTEDAPR 1 1 1972 SHEET 3 OF 3 ELECTROSTATIC PIGMENT FILTER This invention relates in general to filtering and more specifically to a filter for separating suspensions having electrically sensitive solid particles.

Some processes today require the use of suspensions including colloids that have electrically sensitive solid particles suspended in a relatively electrically insulating carrier. It may be beneficial to separate the particles from the carrier for many reasons. It may be necessary to separate the two coming in large quantities for long periods of time, yet use relatively small equipment for the filtering. Recently, a new imaging system was developed which does require such a separating technique. This imaging system utilizes one or more types of photosensitive radiant energy absorbing particles bearing a charge when suspended in a non-conductive liquid carrier and placed in an electroded system. Images are formed when the suspension is exposed to an image radiation configuration. Descriptions of imaging systems involving this phenomena are described in U.S. Pat. Nos. 3,384,565 and 3,384,488 to V. Tulagin et al., 3,383,933 to S. Yeh and 3,384,566 to H. E. Clark all issued on May 21, 1968. The particles of the described systems migrate in image configuration providing a visual image at one or both of two electrodes between which they are placed. The system employs particles which are photosensitive and which apparently undergo a net change in charge polarity or a polarity alteration upon exposure to activating radiation by interaction with one of the electrodes. Mixtures of two or more differently colored particles are used to secure various colors of images and imagining mixes having different spectral responses. Particles in these mixes may have either separate or overlapping spectral response curves and may be used in subtractive color synthesis.

Improvements in the basic system have been developed and such an improvement is the subject matter of U.S. Pat. No. 3,427,242 issued on Feb. 11, 1969. This application discloses means for operating a continuous imaging system which provided for repetitive imaging in a continuous manner with automated apparatus shown schematically. In a system of that type, the electrodes would have to be cleaned of residual ink in order that there not be interference from prior images on those subsequently formed by the imaging system.

In cleaning apparatus it is possible to brush the electrode clean for further use or to run the electrode through a liquid carrier to remove the particle suspension therefrom. An exampleof this latter method of cleaning is shown in copending application Ser. No. 760,31 filed in the name of J. Weigl on Sept. 17, 1968. Here, one of the electrodes, in the form of a web is passed through a carrier which is of the same composition as the carrier of the suspension particles and which removes those particles from the web. The carrier soon becomes loaded with imaging particles and its cleaning ability becomes impeded.

Normal filtering will remove most of the particles but particles will pass through normal filter materials after relatively short periods of use. However, the invention described herein permits the filtering of electrically sensitive particles such as those in an electrophoretic imaging system so that it can function for a considerably extended time without the need for replacement of the material at the system's cleaning station. This is an important feature in an automated type of commercial imaging apparatus which requires long periods between preventive maintenance overhauls while minimizing component size. Other systems are necessary for filtering colloids. These suspensions are not susceptable to normal filtering but can be separated by the method and apparatus described herein. Also, solid suspensions having at least two solids can be separated if their electrical properties are different from each other.

It is, therefore, an object of this invention to continually filter materials used in the cleaning tanks of electrophoretic imaging systems.

Another object of this-invention is to separate electrically sensitive substances from suspensions in which they are maintained.

Yet another object of this invention is to permit recycling of cleaning compositions in imaging apparatus while removing solid particles therefrom. Still another object is to separate and hold solid particles out of liquid carrier suspensions. A further object is to separate and hold solid particles from a suspension.

It may be that other systems exist or will be discovered or invented that require in their operation filtering that have some or enough of the properties of the apparatus and methods described herein that they will be improved by this invention and use of this invention therein is contemplated hereby.

These and other features, and advantages of the present invention are achieved by providing a filter mechanism having a filter material for holding electrically sensitive solid particles moving through the filter mechanism and employing an electrical field within the filter mechanism to improve the ability of the filter material to separate, trap and hold the solid particles.

These and other objects and advantages of this invention will become apparent to those skilled in the art after reading the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of an imaging system incorporating this invention;

FIG. 2 is a full section of a filter mechanism; and

FIGS. 3 and 4 are schematic illustrations of embodiments of filter mechanisms.

Referring now to the figures, FIG. 1 shows photoelectrophoretic imaging apparatus capable of continuously imaging photoelectrophoretic suspensions. It has a rotary transparent electrode, designated numeral 1, having a transparent glass substrate 2 overcoated with an electrically conductive layer 3. Both the glass and the overcoated layer are optically transparent and may be of a structure having tin oxide coated on glass which is commercially available under the name of NESA glass from Pittsburgh Plate Glass Company. Any other similar structure usable within the imaging system is suitable for this invention. Provided in imaging relation to the injecting electrode 1 is the blocking electrode 10 formed as a cylinder and mounted to be in rotating contact with the injecting electrode.

For the purpose of this invention the term injecting electrode refers to the electrode the properties of which apparently inject charges into photosensitive materials activated by electromagnetic radiation under the influence of an electric field. The term photosensitive refers to the properties of a particle which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when exposed to activating electromagnetic radiation. The term blocking electrode refers to the property of an electrode which once contacted by activated photosensitive particles will not inject sufficient charge into them to cause them to migrate from the blocking electrode surface. The term suspension may be defined as a system having solid particles dispersed in a solid, liquid or gas. Nevertheless, the suspensions described in the imaging embodiment are of the general type having a solid dispersed in a liquid carrier. For a more detailed theoretical explanation of the apparent mechanisms of the operations of this imaging process, see the above-mentioned patents, the disclosures of which are incorporated herein by reference.

The blocking electrode 10 is formed of a blocking surface 12 having a high resistivity constant of at least 10' ohm-cm. and a conductive backing member 13. During the imaging process a thin layer of the suspension 14 is formed on the blocking electrode by dispensing through suitable means from a reservoir such as the container 15.

In order to form the images, a stationary mirror 16 is located within the rotary imaging electrode 1 to receive the image projected from a transparent object 18 and to direct the image through an exposure slit 20 onto the surface of the suspension at the area of contact between the two electrodes. The transparency 18 is shown passing under a light source 22. The transparency is shown as a web and is moved synchronously with the rotating imaging and blocking electrode drums to provide a flowing image. The image is projected through a lens 24 for imaging at the intersection of the two electrodes at the slit 26. The image is preferably projected in a plane normal to the surface of the drums so that distortion of the image at the image plane is held at to minimum.

The injecting electrode 1 is grounded by a connection 30 contacting the conductive surface 3 of that electrode. The blocking electrode is connected to a potential source 31 such that a field exists at the interface of the two electrodes with the blocking electrode shown as being positive.

As the two electrodes rotate in the direction shown by the arrows, a flowing image is formed at the interface of the two electrodes. A portion of the suspension 14 will form the transferable suspension image 32 which remains on the imaging electrode 1 while the non-image portion of the suspension, herein referred to by the numeral 34, is carried away from the interface along the surface of the blocking electrode. The developed image 32 on the surface of the imaging electrode is carried on the electrode drum 1 to be contacted by the transfer material 36 which may be an adhesive web or other suitable transfer material. The web 36 moves along a predetermined path through pinch rollers 38 and 40 and around transfer roller 42. The transfer roll brings the web 36 into contact with the surface of the injecting electrode 1 so that the adhesive transfer material can pick the imaging suspension 32 off the surface thereof. The web then passes a fixing unit 44 which securely fixes the image 32 to the web 36.

As the electrodes continue to rotate, the portion of the suspension 34 which does not form part of the image remains on the blocking electrode and is carried to the cleaning station 46. Here, a cleaning member such as brush 48 is positioned within a container 50 built to permit the blocking electrode 10 to enter the upper portion thereof for contacting of the blocking surface 12 by the brush 48. The container holds a quantity of a liquid carrier 52 which may be the same material as the liquid carrier of the suspension 14 or any suitable composition that will cause the particles remaining on the surface 12 to loosen therefrom and be removed by brush 48. The brush is immersed in the liquid 52.

The particle laden liquid 52 is circulated within the container 50 and through the exit port 54 and tubing 56 into the filter mechanism 58 by the action of the pump P-l. After passing through the filter wherein the photosensitive particles are removed, the liquid 52 is returned to the container 50 at the ingress port 60 by the tubing 62. By thus circulating the liquid 52 through the filter mechanism 58 to continually remove the particles from it, the liquid carrier can be used for extended periods of time during the operation of the imaging system without the need for a large container 50 for frequent changes of the carrier within the container.

The structure of filter 58 is shown in section in FIG. 2. It has an upper housing unit 64 with a liquid ingress port 66 drilled therethrough connecting to tubing 56 for bringing the contaminated liquid 52 into the filter mechanism. An egress port 68 is also drilled into the housing 64 and connects to tubing 62 for removal of cleansed liquid 52 back to the cleaning tank 50 of the imaging system. A second housing member 70 is provided at the opposite end of the filter mechanism from the upper housing 64. A cylinder 72 is insertable into circular grooves 73 and 74 within the housing members 64 and 66 respectively. The cylinder 72 is made of Plexiglas which is a thermoplastic poly-(methyl methacrylate)-type polymer manufactured by Rohm & Haas Co. of Philadelphia, Pa. The two housing members and the Plexiglas cylinder form the container of the filter.

The container is held rigid by four threaded metal rods such as the one shown and referred to as rod 76. The rods are held by bolts placed into a threaded hole 77 in the upper housing 64 and pass through a slot 78 in the lower housing 70 to be mated with nut 79. The rods are used for joining the two housings together and pressing the Plexiglass into the grooves within the two housings. By turning the nuts 79 on each of the four rods 76 the container can be properly pieced together. Within the grooves 73 and 74 into which the Plexiglas cylinder is fitted, a suitable sealant is provided to insure that the filter container is leakproof so that none of the liquid contained therein can escape between the cylinder and housing.

Internal to the Plexiglas cylinder 72 is an electrically conductive screen 80 which is a No. 20 copper or brass screen having good electrical conductive properties. The screen fits into a groove 81 in the lower housing member 70 and abuts a portion of the upper housing 64. At the upper housing 64 is an electrical connecting means 82 which is screwed into intimate contact with the screen 80 by a connecting unit 83. The screen 80 is electrically connected to the positive terminal of the source 84 by the screw 83. An electrical terminal 86 is positioned within the lower housing member 70 along what would be the central axis of the Plexiglas cylinder. The electrical connector is screwed down into the housing 70 by screws 87 and by the electrical connecting unit 88 which protrudes through the lower housing 70 for electrical connection with the negative terminal of the potential source 84.

Positionable within the container thus formed by the structure referred to above is a porous filter member 90 which is formed with a conductive metal lower plate 92 and a conductive metal inner wall 93. The filter member is cylindrical and has a diameter less than that of the mesh screen 80. The lower metal plate 92 is electrically connected to the inner metal wall 93 and further abuts the electrical connector 86 making good electrical contact therewith. When the filter mechanism is assembled, the filter member 90 is pressed tight against the electrical contact 86 while remaining spaced from the screen 80 because of the spacer 94 and because of its positioning around the electrical connector 86 at the lower housing 70. A gasket 96 is provided at the upper housing between the ingress and egress ports so that contaminated liquid entering through port 66 must pass through the filter before reaching the egress port 68 for removal from the filter. Drainage stopcocks 98 and 99 are provided within the lower housing 70 for removal of liquid within the filter should that become necessary. The filter member 90 has a metallic upper plate 100 and metallic lower plate 92 with a cardboard like outer wall 101 and suitable filter materials 102 situated between the inner wall 93 and outer wall 101. The inner wall 93 is perforated to permit the liquid 52 to flow into the filter along the entire length thereof. The outer wall 101 is likewise perforated to permit liquid to flow out of the filter member 90 along the entire length thereof.

When the filter mechanism 58 and the pump Pl are in operation, the liquid 52 flows into port 66 and down through the center of the filter member 90. The potential source 84 is activated such that there is a field between the screen 80 and the inner wall 93 of the filter member 90. The particles are electrically sensitive and while flowing through the filter material 102 are separated from the liquid carrier and held on the filter material 102. The electrical field apparently binds the particles into the filter material increasing the capacity for maintaining particles removed from the liquid many fold. An electrical field of approximately 5,000 volts has been found to be effective to increase the capacity of the filter member 90 for not only removing but holding the electrically sensitive particles therein while permitting the electrically insulating liquid to pass through the filter member for recycling to the image system cleaning tank.

As the liquid passes through the filter material 102 from the inner wall 93 to the outer wall 101, it loses its particle contaminate. The action of pump P-l forces the liquid to the egress port 68 where it is removed from the filter container 58. Since the liquid is an electrical insulator, it is unaffected by the field produced within the filter housing.

Any material suitable for removing the imaging particles from the liquid carrier is usable within the filter mechanism described. In the embodiment shown in FIG. 2, a number EP-78 Purolator oil filter made by Purolator Products, Inc. of Railway, New Jersey was found suitable for the purposes disclosed herein. However, other types of filters as well as other materials capable of permitting suspension movement therethrough have been found suitable.

FIG. 3 shows another embodiment of filter mechanism within the scope of this invention. Here the filter housing is formed in a more or less rectangular shape with Plexiglas housing walls on six sides. For purposes of this illustration however, the top housing wall 108 is broken away in order to particularly point out the inner workings of the filter mechanism of this figure. In the end housing wall 110 is an inlet port, not shown, which permits the contaminated liquid to enter the filter housing. At the opposite end wall 112 of the filter mechanism is the outlet port 114 where the cleansed liquid carrier exits for return to the imaging system cleaning tank. A stopcock 1 is provided for drainage of the filter container. The electrical connections to a suitable power source are shown as electrical terminals 116 and 118 at diametrically opposite positions on the filter housing. The location is not critical provided only that the proper commutating system is used to permit the proper field effects between the plates of the filter.

Adjacent the side housing walls 120 and 122 are grooved positioning members 124 and 126. Each member is grooved to hold a plurality of electrical plates such as plate 128. The electrical plates must be spaced from each other sufficiently apart to permit the insertion of filtering material between them.

Every other electrical plate contacts electrical commutating bar 130 which in turn is connected to electrical terminal 116. The remaining intermediate electrical plates contact commutating bar 132 which is connected to terminal 118. Between each set of plates is a quantity of filtering material 134. The plates are preferably screen members permitting easy flow of the liquid and the particles therethrough, the filter material can be any material which will trap and hold the particles while permitting the liquid to flow through the entire filter mechanism. The terminal 116 is shown connected to the positive terminal of a potential source or battery 136 while the terminal 1 18 would be connected to the negative terminal of the source. The terminals may be reversed however without materially affecting the operation of the filter. It can be seen that between every two plates an electric field is produced across the filtering material therebetween. Each of these units is a filter chamber 138. The chambers thus form a series of electrostatic filters for trapping and holding larger quantities of electrically sensitive particles while allowing an electrically insulating liquid to pass through without being affected. Gaskets are positioned to prevent leakage around the entire filter mechanism but are not shown. Bolting mechanisms hold the container together. Such structure is well known. The design of this filter mechanism operates to force the particulate matter successively through fields of opposite signs. Since there is no way to guarantee the sign of charge on the particulate matter passing through the filter mechanism it is beneficial to collect it in reversing fields (spacially reversing) in order to guarantee that both positively and negatively charged particles are removed. This still does not account for neutral particles but since true neutrality is difficult to achieve it is probable that the cyclically reversing fields will remove the majority of the low charge particles and in addition some will be removed by mechanical trappings. This filter can efficiently separate solids from solids and also colloidal suspensions.

FIG. 4 shows a schematic view of another embodiment of a part of an electrostatic filter mechanism. Here, within the housing walls 140 and 142 are a plurality of conductive mesh plates 144 overcoated with a high dielectric material which may be similar to the materials employed on the surface 12 of the blocking electrode 10. This material may be sprayed and baked on the conductive screening material of the plates 144 although the method of creating the plates is unrelated to the invention. When the contaminated liquid enters the housing at the entrance 146 and flows past the plates, the electrically sensitive particles are affected by the electrical field between adjacent plates. The particles are attracted to the coated material where they are held due to the field between adjacent mesh plates. The insulating liquid 52 passes unaffected by the field through the mesh plates and out of the filter mechan'mm. To remove the accumulated particles from the coated plates, the electrical power source is disconnected from the plates and the filter is flushed with a suitable liquid such as the liquid carrier used for imaging. This flushing can be used to remove the particles from the coated plates at selected intervals when the imaging system is not being used. Between flushes, the system can operate as described in the previous figures.

The external power source may be eliminated from the filters discussed hereinabove provided that the flow of the suspension through the filter is of a large enough velocity to generate an electrostatic field between portions of the filter mechanism. This electrostatic field replaces the field applied to the filter mechanism by the potential sources shown and thus operates in the same manner as the electrostatic filter mechanism described above. The difference is that the electrical field is fonned by triboelectric electrostatic attraction generated from the flow of the material rather than an external power source.

While this invention has been described with reference to the structures disclosed herein and while certain theories have been expressed to explain the experimentally obtainable results obtained, it is not confined to the details set forth; and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed is: 1. Apparatus for separating solid particles from a relatively electrically insulating carrier with which they are suspended including a housing having an ingress port for accepting the suspenmeans for moving the suspension through the housing a porous member within said housing comprising a high dielectric material coated on a conductive member,

means for generating an electrical field within the housing capable of attracting the solid particles to the porous member,

an egress port in said housing for removal of the electrically insulating carrier.

2. The apparatus of claim 1 wherein said porous member includes a plurality of spaced apart conductive screens.

3. The apparatus of claim 1 wherein said porous member includes porous dielectric materials sandwiched between spaced apart conductive members coated with dielectric material.

4. The apparatus of claim 3 including a plurality of cells comprising dielectric material sandwiched between two members.

5. The apparatus of claim 1 wherein said means for generating an electrical field includes a plurality of conductive members, an electrical potential source, and commutator means selectively connecting said conductive members to said source such that an electric field exists between said conductive members.

6. The apparatus of claim 1 wherein the dielectric coating is permanently affixed to the conductive member.

7. The apparatus of claim 1 wherein the conductive member is a screen-type mesh.

8. The apparatus of claim 1 including a plurality of porous members.

9. The apparatus of claim 1 wherein the means for generating an electrical field includes electrical connector means for connecting adjacent porous members to opposite polarities of an electrical source.

10. The apparatus of claim 8 wherein the dielectric coating is permanently affixed to the conductive member.

1 1. Apparatus for separating solid particles from a relatively electrically insulating carrier with which they are suspended including a housing having an ingress port for accepting the suspensron,

means for moving the suspension through the housing,

a grooved upper housing member, a grooved lower housing member and an electrically non-conductive chamber insertable between the upper and lower housing members in the grooves therein and sealing means associated with said grooves to form a liquid tight seal thereat.

13. The apparatus of claim 12 wherein said support means include a plurality of rods interfacing with the housing for preventing the suspension materials from escaping therefrom.

* l It t I 

1. Apparatus for separating solid particles from a relatively electrically insulating carrier with which they are suspended including a housing having an ingress port for accepting the suspension, means for moving the suspension through the housing a porous member within said housing comprising a high dielectric material coated on a conductive member, means for generating an electrical field within the housing capable of attracting the solid particles to the porous member, an egress port in said housing for removal of the electrically insulating carrier.
 2. The apparatus of claim 1 wherein said porous member includes a plurality of spaced apart conductive screens.
 3. The apparatus of claim 1 wherein said porous member includes porous dielectric materials sandwiched between spaced apart conductive members coated with dielectric material.
 4. The apparatus of claim 3 including a plurality of cells comprising dielectric material sandwiched between two members.
 5. The apparatus of claim 1 wherein said means for generating an electrical field includes a plurality of conductive members, an electrical potential source, and commutator means selectively connecting said conductive members to said source such that an electric field exists between said conductive members.
 6. The apparatus of claim 1 wherein the dielectric coating is permanently affixed to the conductive member.
 7. The apparatus of claim 1 wherein the conductive member is a screen-type mesh.
 8. The apparatus of claim 1 including a plurality of porous members.
 9. The apparatus of claim 1 wherein the means for generating an electrical field includes electrical connector means for connecting adjacent porous members to opposite polarities of an electrical source.
 10. The apparatus of claim 8 wherein the dielectric coating is permanently affixed to the conductive member.
 11. Apparatus for separating solid particles from a relatively electrically insulating carrier with which they are suspended including a housing having an ingress port for accepting the suspension, means for moving the suspension through the housing, a porous member within said housing, means for generating an electrical field within the housing capable of attracting the solid particles to the porous member, an egress port in said housing for removal of the electrically insulating carrier compressive support means external to said housing for maintaining rigidity of said housing.
 12. The apparatus of claim 11 wherein said housing includes a grooved upper housing member, a grooved lower housing member and an electrically non-conductive chamber insertable between the upper and lower housing members in the grooves therein and sealing means associated with said grooves to form a liquid tight seal thereat.
 13. The apparatus of claim 12 wherein said support means include a plurality of rods interfacing with the housing for preventing the suspension materials from escaping therefrom. 